The present invention relates to a process for forming patterned metallic anti-cavitation layers in a thermal ink jet heater die using positive resist image reversal.
One of the many uses for patterned thin films is in the fabrication of thermal ink jet heater dies for use in printing devices. A thermal ink jet printhead typically comprises two halves which are joined together. A first half comprises a number of electronic circuits including timing gates and addressable resistors (i.e. heating elements or heaters). The second half comprises an ink reservoir and a number of etched channels. When the first and second halves are joined together, an exclusive, addressable heating element rests within each channel. The heating element, when activated, draws ink from the reservoir through the channel and out of a channel orifice or nozzle. Vapor ink bubbles are formed in the channel during this process. Collapse of these bubbles can cause damage to heating elements within a channel. Accordingly, an anti-cavitation material is used in each channel to prevent such damage.
An anti-cavitation material, such as tantalum, has a number of important properties: high strength and hardness, corrosion resistance, and excellent heat and electric conducting properties. Typically, tantalum structures are formed in the ink channels using a deposition and etch technique. As shown in FIGS. 1a-c, with this process a thin film of tantalum 3 is deposited onto a substrate 1 and coated with photoresist forming a photoresist layer 5. The photoresist layer is then exposed to radiation such as electromagnetic radiation or electron beam radiation. For example, the photoresist layer 5 is exposed to Ultra-Violet (UV) light through a mask 7. The photoresist layer is then developed leaving a patterned photoresist layer 5 on the tantalum layer 3. Depending on the type of substrate being used, the tantalum layer 3 is patterned either by wet etching or plasma etching techniques, which are well known in the art. The substrate 1 is then cleaned, thus removing the photoresist.
In a typical thermal ink jet printhead fabrication process, a silicon wafer will be prepared which contains only the first halves of the printhead having electronic circuitry, as described above. Approximately 100 to 600 of these electronic circuitry halves can be placed on a 4-inch diameter silicon wafer. In using the aforementioned deposition and etch technique, the anti-cavitation layer is blanket coated over the entire wafer and all of these printhead halves.
Blanket coating a wafer with a hard material such as tantalum results in a tantalum film which is highly stressed as deposited. This stress can lead to cracking in the film. Any cracks in the film severely degrade the heat and electric conducting properties of the tantalum layer in those areas. By depositing tantalum in isolated regions only where required, this stress cracking can be avoided, thus improving yields.
Electrical isolation is needed between aluminum conductors coupled to the heating elements and the anti-cavitation layer. To facilitate this, Level 7 silicon nitride is deposited as a layer between the aluminum conductors and the anti-cavitation layer. When an anti-cavitation layer such as tantalum is plasma etched during the patterning step, the nitride layer must be made thicker than desired to compensate for tantalum overetching and any non-uniformity in the etching process.
A number of other methods have been utilized for depositing various metals on semiconductor substrates with variable success and certain limitations. For instance, in U.S. Pat. Nos. 3,982,943 and 4,004,044, inner and outer photoresist layers are formed on a substrate with an intervening polymeric separating layer. The separate developing of the two photoresist layers causes a larger aperture in the inner photoresist layer than the outer photoresist layer, thus forming an overhanging structure. After deposition of a metallic film in these apertures, the photoresist layers are easily removed.
In U.S. Pat. No. 4,104,070, a photoresist is modified by adding Monazoline C. The resulting development of exposed areas in a photoresist layer forms clean and protected areas on a wafer.
In U.S. Pat. No. 4,564,584, a second photoresist layer containing imidazole having a negative exposure quality is coated over a developed first photoresist layer. The entire structure is then exposed to light and the undeveloped areas of the first photoresist layer are developed and removed. What remains is the imidazole photoresist covering the areas that were originally developed in the first photoresist layer.
In U.S. Pat. No. 4,284,706, a photoresist layer comprising a phenolic-aldehyde resin, a naphthoquinone diazide sulfonic acid ester sensitizer, and a profile modifying agent is formed on a substrate. When developed, this photoresist layer will have a negatively sloped aperture which improves the deposition of a metallic film.
In U.S. Pat. No. 3,873,361, an organic polymeric photoresist layer is first formed on a substrate and baked. Then, a metallic layer and a second photoresist layer are formed on the organic photoresist layer. The second photoresist layer is exposed through a mask and developed leaving exposed areas on the metallic layer. The metallic layer is then etched leaving exposed areas of the organic photoresist layer. When removing the exposed organic photoresist layer, the aperture through the organic photoresist layer is larger than the aperture through the second photoresist layer. This facilitates an improved deposition of metallic film.
In U.S. Pat. No. 3,934,057, several photoresist layers are formed on a substrate. Each of the layers has a successively slower dissolving rate in the resist developer. When developed, apertures in these photoresist layers have overhanging edges which are useful in metal deposition and lift-off.
None of these methods address the stress-cracking problems seen in depositing an anti-cavitation layer, such as tantalum, onto a thermal ink jet printhead. Many metals, such as gold, silver, copper, and aluminum do not exhibit stress-cracking problems when blanket deposited onto a substrate. There is a need for a deposition method that places anti-cavitation material in appropriate positions in a thermal ink jet printer die while avoiding stress-cracking.